The present invention relates to a device for converting energy of electrical origin and storing energy in the form of compressed air using a liquid environment, in particular the aquatic environment.
The present invention also relates to a method for conversion between the electrical and, respectively, aeraulic forms of energy.
Such a device makes it possible to convert electrical energy easily and efficiently into stored aeraulic energy and vice versa in order to be distributed over the grid. The invention applies in particular to the field of the conversion of electrical energy and the storage thereof once converted into a form capable of long-term storage, in this case the form of compressed air.
The production of electricity is a vital issue of our time. Production technologies based on renewable energy sources have been developed, on the one hand, to diversify the sources for supplying energy and, on the other hand, to produce electrical energy in an ecological manner. However, the need of consumers for electricity does not necessarily coincide with the production of electricity from renewable sources such as solar or wind power. The problem then arises of storing the electrical energy when it cannot be consumed directly by the consumers via the electrical distribution grid. Now electricity can only be stored when converted into another form of energy, then it has to be converted back into electricity when released in order to supply power to the electrical distribution grid when the latter demands it. These essential transformations between forms of energy are subject to energy losses, which should be minimized.
Different energy conversion and storage devices are known, in which the electrical energy is converted then stored in the form of compressed air. The electricity typically originating from solar panels or wind turbines is conveyed to a device located partially on the surface of an aquatic environment and partially submerged in said aquatic environment. The transformation of the electrical energy is carried out using this electricity as an energy source for compressing air, which is sent into a reservoir kept under substantially constant pressure owing to the hydrostatic pressure prevailing at the submerged depth of the reservoir. For the compression, rather than using a compressor driven directly by an electric motor, a system for pumping a liquid which fills, from below, a conversion chamber in which the air to be compressed is trapped in the upper part is preferably used. This manner of compression has the advantage of being quasi-isothermal. The liquid originates from an external environment such as the aquatic environment or originates from a reservoir. The trapped air is compressed up to a predetermined pressure by pumping liquid into the conversion chamber, the liquid forming a liquid piston for compression. The compressed air is then transferred to the reservoir for storing compressed air. Then the liquid is replaced with low-pressure air in the conversion chamber and a new conversion cycle can begin.
When the grid demands electrical energy, the energy stored in the form of compressed air is converted back into electricity. The compressed air of the storage reservoir is transferred to the conversion chamber initially filled with liquid. The compressed air expands in the conversion chamber and pushes the liquid through a dynamo-hydraulic machine, such as a turbine, coupled to an electric generator supplying power to the grid.
However, these devices are deficient in terms of efficiency. The pumps and the turbines work under a very small pressure difference at the start of pumping and at the end of turbining, and thus have to ensure a very high throughput in order for their power to be significant. Conversely, at the end of pumping and at the start of turbining, the difference between the high and the low pressure of the pump or the turbine is great and therefore the throughput is much lower, for a given electrical power. The dynamo-hydraulic machines (pumps or turbines or reversible pump-turbines) cannot be optimized for all of these very varied operating conditions.
The problem is complicated by the need for the conversion of electrical energy into another form or vice versa to be carried out, from an electrical point of view, at a power that fluctuates as little as possible, so as to facilitate the taking or returning of the electrical energy from or to the grid.
The article “Le stockage hydropneumatique d'énergie HyPES” [Hydro-pneumatic energy storage, HyPES] by Dr. Sylvain LEMOFOUET and Prof. Alfred RUFER, Bulletin 9/2011 Electrosuisse pages 38 to 42, describes a mechanical-pneumatic conversion device having a hydropneumatic system comprising several stages with different cylinder capacities. “Each stage is essentially constituted by a hydraulic motor pump with variable throughput, a hydraulic directional module and a compression/expansion chamber with integrated heat exchanger.” This device comprises many elements, its price and maintenance are relatively high, and it is complex to install.
The purpose of the present invention is to overcome, completely or partially, at least one of the above-described problems through a novel device for converting electrical energy into pneumatic energy and vice versa.
A purpose of the invention is to produce a device with a good cost-efficiency ratio.
Another purpose of the invention is to limit the energy losses.
Another purpose of the invention is to propose a device providing electrical energy in such a way that the power fluctuation due to the mode of conversion is small.
Yet another purpose of the invention is to propose a machine comprising a reduced number of parts and/or components.